Method for preventing the leakage of microwave energy from microwave heating oven

ABSTRACT

The object of the present invention is to provide a method for preventing the leakage of microwave energy through the gap between the oven and the door during the operation of the microwave heating device, and the object of the present invention can be attained by placing ferromagnetic material, i.e., a mixture mainly composed of ferrite powder in the path of waveleakage.

United States Patent lshino et al.

[ METHOD FOR PREVENTING THE LEAKAGE OF MICROWAVE ENERGY FROM MICROWAVE HEATING OVEN [75] Inventors: Takeshi Ishino; Nobuyuki Ono, both of Akita-ken, Japan [73] Assignee: T.D.K. Electronics Company, Ltd.,

Tokyo, Japan [22] Filed: June 22, 1970 [2]] Appl. No.: 48,137

[30] Foreign Application Priority Data Jan. 21, 1970 Japan 45/5098 June 26, 1969 Japan 44/50870 [52] U.S. Cl. 219/10.55, 252/6251, 333/98 P [51] Int. Cl. H051) 9/06 [58] Field of Search 333/98 P; 219/10.55;

[56] References Cited UNITED STATES PATENTS 3,511,957 5/1970 Cannon et al. 219/59 X June 26, 1973 3,525,841 8/1970 Haagensen et al 219/10.55 2,956,143 10/1960 Schall 2l9/10.55 2,958,754 11/1960 Hahn 219/l0.55 3,196,242 7/1965 De Vries et al... 219/10.55 3,436,508 4/1969 Fritz 219/10.55 3,242,303 4/ 1966 Crapuchettes 219/ 10.55

Primary Examiner-R. F. Staubly Assistant Examiner-Hugh D. Jaeger Attorney-Burgess, Ryan and Wayne [57] ABSTRACT The object of the present invention is to provide a method for preventing the leakage of microwave energy through the gap between the oven and the door during the operation of the microwave heating device, and the object of the present invention can be attained by placing ferromagnetic material, i.e., a mixture mainly composed of ferrite powder in the path of waveleakage.

2 Claims, 8 Drawing Figures PATENTEDJUNZB I975 SHEUIUFZ FIG. 4

FIG. 3

JM W

Y am

INVENTOR-S @MM /W ,Q a11 H-L-CZZZEYS PATENTEUJUNZS I973 ATTENUATION SHEU 2 [If 2 FIG. 7

ELECTRO- CONDUCTIVE RUBBER 20 4'0 6'0 80 OPENING AREA (/o) INVENTOKS BY I HTTO/QUElS BACKGROUND OF THE INVENTION What is described as microwave heating oven in the specification of the present invention is a device for heating a material with microwaves generated by a microwave generating device, and for raising the temperature thereof within a remarkably short time, and the frequency of the microwave which is used in the device is such a high frequency as 2.45 GHz, as is generally known to those skilled in the art.

The microwave leaks out from a gap, if any, between the microwave heating oven and the door thereof as is shown in the accompanying drawings, and the leaking microwave generates noise in radio receivers or TV sets, and additionally it causes a radio-frequency burn to the human body as is well known to those skilled in the art. Counter measures for preventing the above mentioned leakage of the microwave have been provided in that a plate form metal spring is placed between the main body of the oven and the door and the gap between the door and the main body is eliminated thereby.

However, when such counter measures as above are used, it is very difficult to totally remove the gap between the main body of the oven and the door, and even if this gap should be totally eliminated, the leakage of the microwave may be observed as in the abovementioned case wherein a gap still is present between the main body of the oven and the door, such as if a film of metal oxide be formed on the surface of the metal forming the main body of the oven and the door, or if a foreign matter, such as dust is present between the main body of the oven and the door. The abovementioned microwave is an ultrashort microwave of 2.45 GHz, and therefore the microwave also leaks out when there is a gap between two metals forming the main body of the oven and the door through, or when a gap is present due to insulating material.

In order to prevent the leakage of the microwave as mentioned above, in accordance with the prior art, as is disclosed in US. Pat. No. 2,956,143, a plate form metal spring is placed between the main body and the door, and an electroconductive rubber is used in the form of packing to tightly close the door onto-the main body.

The structure and the properties of the above mentioned electroconductive rubber have not been disclosed in the above mentioned patent, but generally speaking, an electroconductive rubber is prepared by blending natural rubber or synthetic rubber with graphite type carbon. Since the electroconductive rubber has electroconductivity, the electroconductive rubber reflects the microwave in the same manner as metal, and therefore the leakage of the microwave can be prevented. However, rubber is aged when it is used for a long time due to the repetition of heating, cooling and compression thereof, causing it to lose the elasticity thereof, or to be deformed, and there will be a gap between the above-mentioned electro-conductive rubber and the main body or the door as a result of long time use of an oven, thus causing microwave leakage.

The present invention has been developed to remove the above-mentioned drawback, and the present invention provides a method for preventing the leakage of the microwave by absorbing the microwave even when there is a gap of a certain degree between the main body and the door, the absorption of the microwave being achieved by using, in place of the electroconductive rubber for reflecting the microwave, ferromagnetic material having such a property as to absorb the microwave, such as a mixture mainly composed of ferrite powder.

SUMMARY OF THE INVENTION The present invention relates to a method for preventing the leakage of a microwave through a gap between the main body of the microwave heating oven and the door during the operation of said microwave heating oven, and more particularly, to a method for preventing the leakage of the microwave by placing a microwave absorber consisting of ferromagnetic material (i.e., a mixture mainly composed of the powder of ferrite) having suitable size and preferable properties in the path through which the microwave leaks out.

The output of the microwave of the heating oven is different in accordance with the application thereof, i.e., the output of the microwave is different depending on whether the microwave heating oven is for industrial use or home use. As the inventors of the present invention carried out experiments on a microwave heating oven for home use, which is now being commercially distributed, the explanation about the experiments of the home use oven is given in the following paragraphs in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a perspective view of a microwave heating oven for home use;

FIG. 2 FIG. 5 are drawings showing sectional views thereof;

FIG. 3 FIG. 5 being the drawings showing the arrangement of the microwave absorber of the present invention between the main body of the oven and the door;

FIG. 6(a) and FIG. 6(b) are drawings showing the arrangement of the microwave absorber in the waveguide, FIG. 6(a) being a longitudinal sectional view and FIG. 6(b) being a cross sectional view along the line I I of (a);

FIG. 7 is a diagram showing the relation of the opening area A of FIG. 6(b) (i.e., the gap formed by the microwave absorber in the waveguide) and the attenuation of the microwave.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The accompanying drawing, FIG. 1 is a perspective view of the conventional microwave heating oven for home use; FIG. 2 is a drawing showing the longitudinal sectional view of the embodiment of FIG. 1'.

The oven consists of the main body 1 and the door 2 and in order to prevent the leakage of microwave with such a spring means for preventing the microwave leakage was measured, and as a result, from 4 to mW per 1 cm was found at positions 5 in FIG. 1 and FIG. 2, in the neighbourhood of the door 2.

tivity thereof are respectively set to be t and the following relation can be obtained;

H6. 3 FIG. 5 are the drawings showing embodi- 5 =c' j h r in p. and e re respectively the real ments of the present invention, and a microwave absorber 6 is placed in the path through which the micro wave leaks out during the operation of the oven, wherein the path is the gap formed between the main body 1 of the microwave heating oven and the door 2. It is possible to remarkably reduce the amount of the leakage of the microwave by the method of the present invention.

As the above-mentioned microwave absorber, the mixture was prepared by mixing ferrite powder and rubber at the weight ratio (Wt parts) of 5:1, i.e., the mixture was prepared by mixing five parts of the ferrite powder a a one part of the rubber carrier in a dispersed state, and this mixture was used in the present invention. Hereinafter, such mixture is described as rubberferrite.

The above-mentioned ferrite is an inorganic compound having spinel structure represented by the general formula given below:

(wherein M is a divalent metal such as Ni, Cu, Zn, Mn, Mg, etc.) Such a ferrite as mentioned above can be in any composition, but the Ni-Z'n type ferrite wherein M is Ni or Zn is most effective. The particle size of the above-mentioned ferrite powder was from 1 to 5,11,.

On the other hand, in this invention, natural rubber or synthetic rubber can be used as the rubber to be mixed with ferrite powder, and other high molecular compounds such as plastics can be also used in place of the rubber.

The prepared rubber-ferrite having 1 cm of thickness, could give the attenuation of from 7 to 10 db parts of relative permeability and relative permittivity; p. and e" are respectively the imaginary parts thereof;

j is an imaginary number unit).

The preferable microwave absorber to be used in the method of the present invention is a ferromagnetic material having the value of I'll X |i/,u.' X E" of more than 1.25 and the product ofp. and e of more than 9.0 (i.e., x a 6.0).

The following are the reasons for restricting the microwave absorber of the present invention to the ferromagnetic material having the above-mentioned properties.

The ferrite powder to be used in the method of the present invention, can be prepared by forming (pressing) the mixture composed of 20 mol of NiO, 20 mol of ZnO and 60 mol of Fe O calcining the mixture at 1,200 C for 1 hour to prepare Ni-Zn type calcined ferrite, and crushing said calcined ferrite in a ball mill for 5 hours. The powder of ferrite has particle size of from 1 to 5p, and as the organic high molecular compound to be mixed with the above prepared ferrite powder, fluorine-contained rubber [whose trade name is Kel F elastomer] is used, and the above prepared ferrite powder is mixed by the ratio of 0.5, 1.0, 2.0, 4.0, 5.0 and 8.0 parts by weight to one part by weight of the fluorine-contained rubber, and the obtained mixture is sufficiently mixed by using a kneader, and thus six kinds of samples from No. l to No. 6 were obtained.

The values of ,u, ,u", e and e, and the attenuation (db/cm) against 2.45 GI-Iz microwave of the samples of No. l No. 6 were measured, and the following results were obtained.

TABLE Sample No.

Attenuation (db/cm against 2.45 GHZ microwave.

The above prepared rubber-ferrite absorber 6 was The inventors of the present invention found out the fact that there is the following relation between the properties of the ferrite or the mixture of the ferrite powder and the effect thereof as the microwave absorber.

The following is an explanation about the abovementioned relation.

When the relative permeability of the ferrite or the mixture of the ferrite powder and the relative permit- One part by weight of synthetic rubber (Neoprene) and 0.5 part by weight of graphite type carbon (whose particle size is 0.01M) were mixed to prepare the electroconductive rubber whose specific resistance is 10 Referring to FIG. 6(a) and FIG. 6(b), a plate form rubber-ferrite 8 having 6 mm of thickness y or a plate form electroconductive rubber 8 having 6 mm of thickness y were prepared from the above prepared rubberferrite (Sample No. 1 No. 6) or the above prepared electroconductive rubber, and the above prepared plate form rubber-ferrite 8 or the plate form electroconductive rubber 8 were inserted into the waveguide 7 of Model WRJ-2 (5.46 cm X l0.9 cm). The sizes of said plate form rubber-ferrite and said plate form electroconductive rubber were selected in such a manner that the ratio of the opening area (gap A) of the waveguide to the cross sectional area of the waveguide, could be 0, 10, 20,30, 40, 50, 60, and percent, and by using 2.45 Gl-Iz microwave, the amount of attenuation of the microwave in the case of the abovementioned opening areas were measured, and the values as shown by the curve of FIG. 7 could be obtained.

As is apparent form the above-mentioned curve of FIG. 7, in the case of electroconductive rubber, the amount of attenuation is abruptly reduced as the opening areas are increased, and when there is even a slight gap, the effect for preventing the leakage of the microwave is deteriorated.

As is shown by the curve of FIG. 7, Sample No. 6 among the above prepared rubber-ferrites, is the most effective, and the effect of Sample No. l is the poorest.

From the above given table and the curve, as the fer romagnetic material for preventing the leakage of microwave, those having p. X e of more than 9 and the value of U11 X ll/p. X e of more than 1.25 are preferable.

In regard to the manner in which the rubber-ferrite is applied, there are a number of forms in addition to v the manner shown in FIG. 3, for example, as shown in FIG. 4 and 5.

Measurements were carried out on the forms shown in FIG. 4 and FIG. 5, and almost the same results could be obtained as in the embodiment shown in FIG. 3.

The rubber-ferrite shown in 6 of FIG. 4 includes copper net (whose mesh is 1 mm), and the matter shown in 7 of FIG. 5 is an electroconductive rubber, and thus, the inventors of the present invention found out the fact that the effect of microwave absorption can be increased by using additional means in addition to the rubber ferrite alone.

It has been found out that most of the leaking microwave can be absorbed by the rubber-ferrite placed in the path through which the microwave passes, regardless of forms or structure of the rubber-ferrite, if said rubber-ferrite has a certain size.

In order to place the rubber-ferrite as the above mentioned microwave absorber, in the path through which the microwave of the heating oven passes, it may be fixed on the main body of the oven or on the door, and as a method for fixing the above-mentioned wave absorber, adhesion by means of adhesive, or fixation by means of fixing tools, or the like is used.

We claim:

1. A method for preventing the leakage of microwave energy from a microwave heating oven comprising placing ferromagnetic material as a microwave absorber in the path through which the microwave leaks out, between the main body of the microwave heating oven and the door thereof, said ferromagnetic material being a mixture of the powder of ferrite and an organic high molecular compound, said ferrite having the formula:

wherein M is a divalent metal such as Ni, Cu, Zn, M'n or Mg, and said organic high molecular compound being selected from a group consisting of natural rubber and synthetic rubber.

2. A method according to claim 1 wherein said ferromagnetic material is a mixture prepared by mixing of 2 to 8 parts by weight of the powder of ferrite to one part by weight of said organic high molecular compound. 

2. A method according to claim 1 wherein said ferromagnetic material is a mixture prepared by mixing of 2 to 8 parts by weight of the powder of ferrite to one part by weight of said organic high molecular compound. 